单项选择题

Since there is such an abundance of food in the sea, it is understandable that some of the efficient, highly adaptable, warm-blooded mammals that (67) on land should have returned to the sea. Those that (68) have flourished. Within about 50 million years-- (69) time at all, geologically speaking--one of the four kinds of mammals that has (70) to a marine environment has developed into the largest of all animal (71) , the whale. A second kind, the seal, has produced what is probably the greatest population of large carnivorous mammals on Earth.
This suggests that these "top dogs" of the ocean are (72) and multiplying. (73) , such has not been the case, at ieast not for the last 150 years. Trouble has closed in (74) these mammals in the form of equally warm-blooded and even more (75) adaptable predators, humans. At sea, (76) on land, humans have now (77) themselves on the top of the whole great pyramid of life, and (78) have caused serious problems for the mammals of the sea, There is a simple (79) for this.
(80) mammals have the misfortune to be swimming aggregates of (81) that humans want: fur, oil and meat. Even so, they might not be so (82) to human depredation if they did not, like humans, (83) so slowly.
Every year humans (84) more than 50 million tons of fish from the oceans without critically depleting the population of any (85) . But the slowbreeding mammals of the sea have been all but wiped out by humans (86) satisfy their wants and whims.